1. Field of Invention
This invention relates to a method of identifying a particular optical cable out of a number of similar optical cables at any point along the route of installation.
2. Prior Art
In modern telecommunication systems, optical cables are housed in ducts and conduits and laid along the route of installation.
With the recent development of telecommunication networks, a large number of different optical cables are often laid along a single route of installation.
Particularly in the case of an optical LAN system installed in a so-called intelligent building, a large number of optical cables are often housed in a single duct to produce a congested condition there, anticipating an increase in the number of subscribers of telecommunication services in the future. Optical cables are accompanied by greater possibilities than ever of branching and replacing operations after the initial installation.
FIG. 9 of the accompanying drawings schematically illustrates a typical telecommunication network comprising central stations 01, relay stations 02 and terminal stations 03 interconnected by means of a large number of optical cables C, which are often ramified, star-connected and looped.
Since many of these optical cables resemble to one another, skilled workers find it difficult to single out a particular optical cable at a given intermediate point that needs to be branched or replaced out of a number of cables that have been laid in a same route of installation.
In an attempt to remedy this problem, LaidOpen Japanese Patent Nos. 2-230105 and 2-230106 propose methods of identifying a particular optical cable by utilizing fluctuation of polarized light in a single mode optical fiber.
Referring to FIG. 10, with these known methods, polarized light is transmitted through a single mode optical fiber of an optical cable C from a light source 1 of a light source station while an external signal (such as mechanical vibration) is applied to the optical cable C at a work site by means of a signal application device 2 so that any fluctuation in the level of polarized light caused by the external signal may be detected to identify the optical fiber by a photodetector 3 as the light is received by a light receiving device 4 at a detecting station.
More specifically, while the operator of the light source station applies a given signal to a particular optical fiber of each of the optical cables in the route for signal transmission and the operator of the detecting station monitors the reception of the signal, the operator in the work site sequentially applies an external signal to the optical cables laid there on a one by one basis and, each time an external signal is applied to an optical cable, the operator at the work site and his colleague at the talk over through a radio communication channel, using, for instance, a pair of transceiver to make sure if the optical cable in question is detected or not.
As the operation of sequentially applying an external signal to the optical cables laid in the work site proceeds on a one by one basis, the operator there will eventually come across the optical cable in question to apply a signal to it and the operator monitoring the signals at the detecting station will detect the cable carrying a fluctuated signal.
Then, the operator at the detecting station notifies his colleague at the work site that the cable to which an external signal is applied last time is the optical cable to be detected so that the latter can identify the optical cable.
Now, the optical cable is identified and the operator on the work site can proceed to a predetermined work to be conducted on the cable in question. The above described known method is advantageous in that it reduces the workload of operators and does not damage optical cables when compared with the local detection method and the local injection method where cable sheaths need to be dismantled to identify a particular optical cable.
The above described known method is used better with the photoelastic effect than with the Faraday effect in terms of the dimensions of the equipment involved and the required capacity of the power source. However, the above described known method of identifying an optical cable by utilizing the fluctuation of polarized light wave is accompanied by certain problems to be solved particularly when it is used with the photoelastic effect. Some of the problems will be described below.
Firstly, it is rather difficult to find an appropriate spot located closest to the active optical fiber of the optical cable in question (which is currently transmitting optical signals) for externally applying signals to that active optical fiber because no one can tell where the active optical fiber can be found from outside.
In many occasions, consequently, the spot selected for applying signals can be remote from the active optical fiber and the objects existing between them within the optical cable can hinder the transmission of mechanical vibration, absorbing the energy of vibration of the applied external signal and lowering the SN ratio to a level below the minimum level (20 dB) required for a signal detector.
If such is the case, the fluctuation in the level of the optical signal detected by the signal detector at the detecting station can be very small and hardly discriminated from noise, leading the operator observing the monitor at the detecting station to commit errors in recognizing the fluctuated signals.
Secondly, a phenomenon of cross talk can take place in adjacent optical cables when the frequency of the external signal (mechanical vibration) applied to an optical cable is low.
This means that the applied external signal is transmitted through not only the optical cable to which it is applied but also any of the optical cables laid in its vicinity to produce a condition where identification of cables is impossible or misidentification of cables can occur.
This problem may be partly avoided by observing higher harmonics in addition to the fundamental wave to identify the external signal.
However, the higher harmonics are not attenuated to a nil level if they leak from the optical cable in question and go into adjacent optical cables and, therefore, the utilization of higher harmonic cannot provide a perfect solution to the above described problem.